The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device. A reductant or diesel exhaust fluid (“DEF”) is typically sprayed or injected into hot exhaust gases upstream of the SCR device. The reductant may be an aqueous urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is absorbed by the SCR device. The ammonia then reduces the NOx to nitrogen in the presence of the SCR catalyst. In the event reductant is not present, or if the reductant is of relatively low quality, then the SCR device may not be able to efficiently reduce the NOx to nitrogen. The quality of reductant generally pertains to the percentage and the purity of the urea present in the reductant.
Government regulations require the detection of low quality reductant. Specifically, the detection of low quality reductant is correlated to emissions (i.e., the level of NOx in the exhaust gas emitted to the atmosphere). Detection is required shortly after start of vehicle operation. In one approach to detect low quality reductant, NOx sensors may be placed in the exhaust gas stream to indicate efficiency of the SCR device. Specifically, a NOx sensor is placed upstream of the SCR device and another NOx sensor is placed downstream of the SCR device. However, this approach may not always accurately determine the quality of the reductant. This is because other factors, such as a deteriorated SCR substrate, or the inability to deliver the reductant to the SCR device may also affect the efficiency of the SCR device. Accordingly, it is desirable to provide a system and method of determining reductant quality.